A fuel cell has been proposed as a clean, efficient, and environmentally responsible power source for electric vehicles and various other applications. One example of the fuel cell is a Proton Exchange Membrane (PEM) fuel cell. The PEM fuel cell includes a membrane-electrode-assembly (MEA) that generally includes a thin, solid polymer membrane-electrolyte having an electrode with a catalyst on both faces of the membrane-electrolyte.
The PEM fuel cell generally has porous conductive materials, also known as gas diffusion media, which abut the anode and cathode electrode layers and distribute fuel and oxidant gases. Fuel, such as hydrogen gas, is introduced at the anode where it reacts electrochemically in the presence of the catalyst to produce electrons and protons. The electrons are conducted from the anode to the cathode through an electrical circuit connected therebetween. Simultaneously, the protons pass through the electrolyte to the cathode where an oxidant, such as oxygen or air, reacts electrochemically in the presence of the electrolyte and catalyst to produce oxygen anions. The oxygen anions react with the protons to form water as a reaction product.
The MEA is generally interposed between a pair of electrically conductive contact elements or bipolar plates to complete a single PEM fuel cell. Bipolar plates serve as current collectors for the anode and cathode, and have appropriate channels and openings formed therein for distributing the gaseous reactants (i.e., H2 and O2/air) over the surfaces of the respective electrodes.
In practice, however, PEM fuel cells are not individually operated. Rather, PEM fuel cells are connected in series, or stacked one on top of the other, to form what is usually referred to as a fuel cell stack. PEM fuel cell stacks are generally loaded in compression in order to maintain low interfacial electrical contact resistance between the bipolar plates, the gas diffusion media, and the catalyst electrode. The low interfacial contact resistance in a PEM fuel cell stack is directly related to the compression loading. Typically, compression loads on the bipolar plate range from about 50 to about 400 psi, and are controlled by a compression retention system. Importantly, such systems are often installed under an even higher building load to compensate for loss in compression that occurs when the building load is removed.
Compression retention systems are typically designed in a manner effective to offset strains produced by membrane swelling and compressive stress relaxation in the fuel cell stack. Such systems act to minimize an over-compression and damage of gas diffusion media in the fuel cell stack, as well as maintain the stack compression and contact pressure between bipolar plates, gas diffusion media, and catalyst layers. It is disclosed in U.S. Pat. No. 5,484,666 that conventional compression systems have consisted of tie rods extending through and between end plate assemblies secured with fastening nuts. Springs threaded on the tie rods and interposed between the fastening nuts and the end plates have been used to apply resilient compressive force to fuel cell stacks in the stacking direction.
In addition to compression retention systems, conventional PEM fuel cell assemblies include delivery subsystems and componentry for distribution of hydrogen fuel, oxidant and coolant to the fuel cell stack. For example, devices such as manifolds with ports for directing gases and fluids to the interior of the stack are common. Subsystems for exhausting reaction products and coolant are also generally present. Further found within fuel cell systems are current collectors, cell-to-cell seals, insulation, pumps, fans, valves, compressors, associated plumbing, electrical connections, and instrumentation. Such subsystems and devices are volume consuming, can represent increased thermal mass when located outside the stack (requiring more time to warm the stack to appropriate temperatures), and in some cases can cause an electrically parasitic load on the fuel cell stack.
Peripheral preconditioning devices and componentry have also been necessary for optimum operation and performance of a fuel cell stack. Such systems can include, for example, reformers for extracting usable hydrogen fuel from hydrogen-containing feedstock. Additionally, humidifiers for wetting the PEM layers of the fuel cell stack and facilitating conduction of protons from the anode layers to the cathode layers of the MEA are often necessary. These peripheral devices require extensive additional hardware which can also lead to poor system efficiency. This poses problems in many applications, such as vehicular applications, where it is desirable that weight and size of a fuel cell system be minimized.
There is a continuing need for a fuel cell system that has an optimized mass and volume and that can be manufactured with an optimized amount of componentry.